1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device which divides a pixel into a plurality of sub-pixels for displaying images in multi-gradation.
2. Description of the Related Art
As a method of displaying images in multi-gradation in a liquid crystal display device, there is known a method of dividing a pixel into a plurality of sub-pixels.
An example of such a method is suggested in Japanese Unexamined Patent Publication No. 2001-34232 (A).
FIG. 1 is a block diagram of a liquid crystal display device 200 suggested in the Publication.
The liquid crystal display device 200 is comprised of a color liquid crystal panel 212, a backlight unit 214, a data processor 216, a driver 218 for driving the color liquid crystal panel 212, and an interface (IF) 222.
FIG. 2A is a partially enlarged view of a display screen of the color liquid crystal panel 212.
As illustrated in FIG. 2A, R, G and B pixels are horizontally arranged in this order in a display screen of the color liquid crystal panel 212 in accordance with a color filter. Colored images are displayed by R, G and B image data through those R, G and B pixels. Black-and-white image is displayed in the liquid crystal display device 200 as follows.
In the liquid crystal display device 200, black-and-white image is displayed with R, G and B pixels being used as a single unit pixel. Since a unit pixel is comprised of R, G and B pixels, the number of brightness displayable in a unit pixel is three times greater than the number of brightness displayable in each of R, G and B pixels.
In other words, a gradation in a displayed image can be made smaller by setting a range between the above-mentioned brightnesses into one-third.
For instance, it is assumed that a unit pixel P is divided into three sub-pixels p1, p2 and p3, as illustrated in FIG. 2B. If each of the sub-pixels p1, p2 and p3 displays images in eight bits, a displayable brightness in each of the sub-pixels p1, p2 and p3 is in the range of 0 to 255 both inclusive, and a displayable brightness in the unit pixel P is in the range of 0 to 765 (255×3) both inclusive. Among the displayable brightness, the minimum brightness 0 is associated with a minimum among image data, and the maximum brightness 765 is associated with a maximum among image data. This ensures that images are displayed with high gradation.
When the data processor 216 supplies a brightness converted from image data, to the unit pixel P, the data processor 216 distributes the brightness almost equally to the sub-pixels p1, p2 and p3.
Specifically, assuming that 8-bit image data is input into a color display unit which displays images in 8-bit, the image data consists of 0 to 255, and a minimum 0 among the image data is associated with a minimum brightness 0 of the color display unit, and a maximum 255 among the image data is associated with a maximum brightness 765 of the color display unit.
Then, the data processor 216 distributes a brightness obtained based on the image data, to the sub-pixels p1, p2 and p3 in accordance with Table 1 shown below. For instance, when a brightness is equal to 0, (0, 0, 0) is assigned to the sub-pixels p1, p2 and p3, when a brightness is equal to 1, (0, 0, 1) is assigned to the sub-pixels p1, p2 and p3, and when a brightness is equal to 2, (0, 1, 1) is assigned to the sub-pixels p1, p2 and p3. The assignment of a brightness to the sub-pixels p1, p2 and p3 is carried out in the same way for a brightness 0 to 765.
TABLE 1BrightnessSub-pixel p1Sub-pixel p2Sub-pixel p3 0000 1001 2011 3111 4112 5122............762254254254763254254255764254255255765255255255
In Table 1, a brightness indicates a gradation to be input into the liquid crystal display device 200.
As illustrated in FIG. 2B, in the liquid crystal display device 200, a pixel is divided into the sub-pixels p1, p2 and p3 which are equal to one another, and the number of gradation is made about three times greater by summing gradation (data to be input into a driver) of the sub-pixels p1, p2 and p3.
Specifically, as illustrated in FIG. 3, input gradation in the liquid crystal display device 200 (that is, data to be input into a driver of each of the sub-pixels) and a brightness which is shown as a standardized brightness in FIG. 3 have a linear relation to each other. Accordingly, a sum of brightness of the sub-pixels p1, p2 and p3 is equal to a brightness of the pixel P.
However, since gradation to be input into the sub-pixels p1, p2 and p3 and brightness of the sub-pixels p1, p2 and p3 are designed to have a linear relation to each other, the number of gradation which the pixel P can accomplish is equal at maximum to 3M wherein M indicates the number of gradation which each of the sub-pixels p1, p2 and p3 can accomplish.
For instance, if each of the sub-pixels p1, p2 and p3 can accomplish 256 gradation, the pixel P consisting of the sub-pixels p1, p2 and p3 could accomplish 766 gradation.
Accordingly, it is not always possible for the conventional liquid crystal display device 200 to display images in desired multi-gradation.
Frame rate control (FRC) makes it possible to display images in desired multi-gradation.
Herein, in accordance with frame rate control, for instance, 10-bit image data is divided into four 8-bit image data, and the thus divided 8-bit image data is successively displayed at an increased frequency. This results in that image data is displayed in 10-bit.
Though multi-gradation can be readily accomplished by frame rate control, frame rate control is accompanied with a problem that flicker much occurs in images displayed in accordance with frame rate control.
Frame rate control is accompanied further with a problem that when frame rate control is carried out at a longer period than a displayed-frame rate, it would not be possible to display moving images in subtle colors or to properly display images in additional gradation.
In order to eliminate flicker, or in order to properly display moving images in designed colors, it would be necessary to raise a frame frequency to switch displaying images at a high rate. However, it is difficult to switch image-displaying at a high rate, because a driver IC of a monitor or a monitor itself has a limited response rate.